Detection of access bursts in a random access channel

ABSTRACT

A technique for detecting one or more access bursts ( 112 ) in a random access channel ( 110 ) is described. Each access burst includes a transmission preamble, the transmission preamble being a member of a preamble set including sequences of preamble symbols that can be obtained by cyclically shifting a basic preamble sequence. A detector ( 118, 120 ) in a radio base station ( 108 ) determines correlation information indicative of a correlation of a single correlation preamble with each of the one or more received transmission preambles, wherein the correlation preamble is a member of the preamble set. The one or more access bursts are detected based on the correlation information.

FIELD OF THE INVENTION

The invention generally relates to random access procedures performed incellular radio networks. More specifically, the invention relates to atechnique for detecting one or more access bursts in a random accesschannel.

BACKGROUND OF THE INVENTION

Today, not only mobile telephones but also other mobile devices such asPDAs (Personal Digital Assistants), notebooks, etc., exchange data withwireless networks via radio interfaces. Typically, a radio base stationof a network, e.g. a mobile network, serves the mobile device by routingdata received from the device through the network towards the recipient,and by transmitting data received from the network side over the radiointerface towards the mobile device.

The transmission resources available over the radio interface, such asfrequency (bandwidth), time (timeslots available in transmission frames)and transmission power, are generally limited and therefore have to beused as efficiently as possible. In this respect, the base stationcontrols not only the resource parameters for downlink transmissions(from the base station to the device), but also for the uplinktransmissions (from the device to the base station). For the uplink, thebase station has to ensure that the mobile device is synchronized withthe transmission scheme of the radio interface with appropriate accuracyto avoid waste of resources. To this end the base station analyzesreceived uplink signals, derives appropriate adjustment values for theuplink transmission parameters used by the device and sends informationindicating the necessary adjustments towards the mobile device, whichthen has to adjust its transmission parameters accordingly.

As an example, the radio base station determines timing misalignmentsbetween the mobile device and the radio base station. Timingmisalignments are caused by the variable propagation round trip delayresulting from a changing distance between mobile device and basestation as well as from the mutual drift between the clocks in the basestation and the device.

Whereas the synchronization of the mobile device may be performed in astraight-forward manner in case of an established uplink connection,during which signals from the device are continuously received andanalyzed at the base station, no such analysis is possible in case thedevice wants to connect for the first time (for example at power-up orduring a handover) or from a standby status (in which the device onlylistens to the downlink). In these circumstances a random accessprocedure has to be performed to achieve synchronization.

In networks such as mobile GSM or UMTS networks, a physical randomaccess channel (RACH) is provided by the base station (also calledNode-B in UMTS) over the radio interface which allows a mobile device toperform a random access procedure. During this procedure, the mobiledevice transmits a specific access burst (as opposed to normaltransmission bursts) in the RACH. In case of a successful detection andanalysis of the access burst, the base station responds by transmittingproper adjustment parameters to the mobile device.

When transmitting the access burst, the uplink transmission parameterssuch as time, frequency and power are in general not accurately alignedwith the transmission scheme supported by the radio base station.Therefore additional resources have to be provided to the random accesschannel to allow for misalignments and avoid interference of the randomaccess bursts with well synchronized normal bursts transmitted, forexample, in neighbouring time slots. These extra resources comprise, forexample, guard periods and guard bands in the time and frequencydimension, respectively.

In GSM networks, a particular RACH time slot is defined in the timedomain. For example, time slot or sub-frame 0 in each radio frame may bereserved for the RACH. In this way, the RACH is orthogonal to other datachannels, e.g. traffic channels. Within a RACH, collisions may occur asmultiple mobile devices may simultaneously request access. In GSM, atmost one of the simultaneously received access bursts can besuccessfully detected, the other bursts therefore remain unanswered bythe base station. A contention resolution scheme may thus include arandom back off procedure, wherein the mobile devices repeat theiraccess requests after a randomly determined time period in case of noresponse from the base station.

An access burst may contain a “preamble” or “signature” sequence, whichis basically a sequence of symbols. Each of the symbols in turn maycomprise a sequence of bits, e.g. 4 bits. Different preambles may beprovided to the mobile devices to allow simultaneous access requests ofmultiple devices in the same cell. An access requesting device isexpected to choose (e.g., randomly) one of the predetermined preambles.The detection of access bursts in the base station thus relies onsearching for the occurrence of any one of the predetermined specificpreamble sequences in the RACH. A specific preamble detector may beprovided in the base station which comprises a number of digitalfilters, one filter for each of the allowed preamble sequences. In casea signal received in the RACH matches with one of the filters to atleast a predetermined accuracy, an access burst can successfully bedetected.

As an example, six different preambles may be used for the accessprocedure. In this case, six filters have to be provided in the preambledetector. Any signal received in a random access channel has to beanalyzed by all six filters in parallel in order to determine if none,one or more access bursts have been transmitted. It is clear alreadyfrom this simple example that the detector requires a highly complexcircuitry including a plurality of digital filters operating in parallelin order to analyze the received signal. Generally, with an increasingnumber of admissible preamble sequences to detect, the number of filtersto be provided and thus the computational complexity increases further.

The upcoming successor of the current UMTS standard called LTE (LongTerm Evolution) will utilize OFDMA (Orthogonal Frequency DivisionMultiple Access) as an orthogonal transmission scheme. Also in thissystem, there will be mutual interference between access burstssimultaneously transmitted by different mobile devices. At the sametime, presumably the number of simultaneous access attempts to beprocessed in parallel will increase and thus the computationalcomplexity of the detector.

In non-orthogonal systems such as W-CDMA, the RACH is shared with otheruplink channels. Here, the preamble detector has to cope with mutualinterference not only between multiple access bursts, but also betweenaccess bursts and other bursts, e.g., normal bursts. Also in thisscenario, a detection of access bursts with an appropriate confidencelevel requires a very complex detector.

There is thus a need for an efficient technique for detecting one ormore access bursts in a random access channel which also allowsconstruction of detectors with limited complexity.

SUMMARY OF THE INVENTION

According to a first aspect of the invention, a method for detecting oneor more access bursts in a random access channel is proposed, whereineach access burst includes a transmission preamble, the transmissionpreamble being a member of a preamble set including sequences ofpreamble symbols that can be obtained by cyclically shifting a basicpreamble sequence. The method comprises the steps of receiving, in therandom access channel, the one or more access bursts each including itstransmission preamble; determining correlation information indicative ofa correlation of a single correlation preamble with each of the one ormore received transmission preambles, wherein the correlation preambleis a member of the preamble set; and detecting, based on the correlationinformation, the one or more access bursts.

The random access channel may be orthogonal or non-orthogonal to otherchannels provided over a radio interface. The preamble set may comprise,for example, 2, 4, 8, 16, 32 or 64 preamble members. A preamble sequencemay comprise a plurality of symbols, for example 449 or 863 symbols. Thecyclic shift may comprise one or more of the preamble symbols. Forexample, the preambles of a preamble set may be obtained or generated bycyclically shifting a basic preamble by one or two symbols or by thefloor function of the preamble sequence length divided by the number ofpreambles in the preamble set. The correlation preamble may be anypreamble of the preamble set. For example, the correlation preamble maybe the basic preamble used for generating the preamble set.

For determining the correlation information, the correlation preambleand the one or more transmission preambles may be treated as periodicsignals. Assuming that a transmission preamble contained in a receivedsignal is periodically continued (which may or may not include a cyclicprefix) and ignoring other parts of a received access burst and otheraccess bursts, signals corresponding to the correlation preamble and thetransmission preamble may be matched over one fundamental period byshifting them against each other in the time dimension.

In one mode of the invention, the step of determining the correlationinformation may comprise cyclically correlating the correlation preambleand the one or more transmission preambles. Such a correlation may beperformed in a frequency domain. In an implementation of this mode ofthe invention, the cyclic correlation is performed using FourierTransform techniques.

In a variant of the invention, the method further comprises the step ofdetermining, based on the correlation information, a propagation roundtrip time for at least one of the one or more received transmissionpreambles. The round trip time may then be used to provide timingadvance values for a proper time alignment to the mobile devices fromwhich the access bursts originate.

The sequences of the preamble set may be cyclically shifted versions ofeach other, which are shifted by at least a minimum shift whichcorresponds to a predetermined maximum time delay for the transmissionpreambles. The time delay depends on channel propagation properties. Forexample, the time delay may comprise a maximum propagation round triptime delay depending on cell size as well as channel impacts such as adelay spread due to multipath propagation. For example, a minimum shiftmay amount to one or more preamble symbols.

In one implementation of the invention, each of the preamble symbolsutilized for the preamble sequences is utilized only once per preamblesequence. In this way, the preamble sequences may comprise essentiallyideal periodic autocorrelation properties, i.e. the autocorrelationfunction is essentially a Dirac function and spurious correlationsignals anywhere else are minimized. A preamble set based on such anideal autocorrelation function may lead to clear correlation indicationswhen correlating the correlation preamble and the one or moretransmission preambles, such that a detection of access bursts can beperformed with high confidence.

The correlation information may comprise a correlation functionindicating the correlation of the correlation preamble and the one ormore transmission preambles in a time dimension. In one mode of theinvention, the step of determining the correlation information mayfurther comprise subdividing the correlation function into zones,wherein each zone is associated with the correlation of one member ofthe preamble set with the correlation preamble. In a variant of thismode, the step of determining the correlation information furthercomprises the steps of determining a correlation peak in the correlationfunction corresponding to one of the one or more transmission preambles;determining the zone within which the correlation peak is located; anddetecting one of the one or more received access bursts based on thedetermined zone. This sequence of steps may be performed multiple times,i.e. corresponding to the number of correlation peaks in the correlationfunction which can be successfully determined. For example, in case thecorrelation function comprises three detectable correlation peaks, thesequence of steps may be performed three times, leading to the detectionof three access bursts corresponding to the correlation peaks.

Each of the zones corresponds to a particular time shift required tomatch the correlation preamble and the transmission preamble. Forexample, a preamble sequence set may contain six preambles, such thatthe correlation function may have six zones. In case all sixtransmission preambles are received in a RACH, there would be acorrelation peak in each zone. In other words, in case several accessbursts are received in the random access channel, several correlationpeaks may be detected which are located in different zones as long asthe originating mobile devices have utilized different preamblesequences of the preamble set.

The method may further comprise the steps of determining a peak shift ofthe correlation peak relative to a border of the zone within which thecorrelation peak is located; and calculating, based on the peak shift,the propagation round trip time. For example, a peak located near to aborder of the zone indicates that the originating mobile device is nearto the base station or near to the cell border.

In one mode of the invention the method comprises the steps ofperforming a Fourier Transform of a received signal; and extracting afrequency band corresponding to the random access channel. Otherfrequency bands may be extracted which correspond to different channels,e.g. signalling or traffic channels.

According to another aspect of the invention, a computer program productcomprising program code portions for performing the steps of any one ofthe method aspects described herein when the computer program product isexecuted on one or more computing devices, for example a detector in aradio base station such as a Node-B in an UMTS or LTE network. Thecomputer program product may be stored on a computer readable recordingmedium, such as a permanent or re-writeable memory within or associatedwith a computing device or a removable CD-ROM or DVD. Additionally oralternatively, the computer program product may be provided for downloadto a computing device, for example via a network such as the Internet ora communication line such as a telephone line.

According to a further aspect of the invention, a detector for detectingone or more access bursts in a random access channel is proposed,wherein each access burst includes a transmission preamble, thetransmission preamble being a member of a preamble set includingsequences of preamble symbols that can be obtained by cyclicallyshifting a basic preamble sequence. The detector comprises a receptioncomponent adapted to receive, in the random access channel, the one ormore access bursts each including its transmission preamble; acorrelation component adapted to determine correlation informationindicative of a correlation of a single correlation preamble with eachof the one or more received transmission preambles, wherein thecorrelation preamble is any member of the preamble set; and a detectioncomponent adapted to detect, based on the correlation information, theone or more access bursts. The correlation component in the detector maybe adapted to treat the correlation preamble and the one or moretransmission preambles as periodic signals.

According to a still further aspect, a radio base station adapted forperforming a random access procedure is proposed, which comprises adetector according to the above-described aspect of the invention. Inone variant of the invention, the radio base station comprises two ormore detectors, wherein the detectors are configured for differentpreamble sets.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following, the invention will further be described with referenceto exemplary embodiments illustrated in the drawings, in which:

FIG. 1 is a schematic illustration of an embodiment of a communicationsystem;

FIG. 2 is a functional block diagram schematically illustrating anembodiment of a detector for detecting access bursts in a RACH;

FIG. 3 is a functional block diagram schematically illustrating afurther embodiment of a detector;

FIG. 4 is a flow diagram schematically illustrating steps of anembodiment of a method for detecting one or more access bursts in arandom access channel;

FIG. 5 is a schematic illustration of a time-frequency mapping of aRACH;

FIG. 6 is a schematic illustration of an access burst format;

FIG. 7 is a schematic illustration of a preamble set with cyclicallyshifted preamble sequences;

FIG. 8 is a schematic illustration of preamble sequencecross-correlations of a preamble set; and

FIG. 9 is a schematic illustration of a shift of a correlation peak dueto propagation round trip time delay.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the following description, for purposes of explanation and notlimitation, specific details are set forth, such as specifictransmission schemes, particular communication nodes and devices, etc.,in order to provide a thorough understanding of the current invention.It will be apparent to one skilled in the art that the current inventionmay be practised in other embodiments that depart from these specificdetails. For example, the skilled artisan will appreciate that thecurrent invention may be practised with non-orthogonal transmissionschemes instead of the examples discussed below, which are based on anorthogonal transmission scheme. The invention may be practised in anywireless network in which a random access procedure is performed. Thismay include also wireless local area networks, for example HIPERLANnetworks, as opposed to cellular telecommunication systems ortelecommunication radio networks.

Those skilled in the art will further appreciate that functionsexplained hereinbelow may be implemented using individual hardwarecircuitry, using software functioning in conjunction with a programmedmicroprocessor or a general purpose computer, using an applicationspecific integrated circuit (ASIC) and/or using one or more digitalsignal processors (DSPs). It will also be appreciated that when thecurrent invention is described as a method, it may also be embodied in acomputer processor and a memory coupled to a processor, wherein thememory is encoded with one or more programs that perform the methodsdisclosed herein when executed by the processor.

FIG. 1 schematically illustrates an embodiment of a communication system100 including multiple mobile devices 102, 104, 106 and a radio basestation 108. The mobile devices and the base station may exchange datawith each other via a radio interface 110. The radio base station 108may belong to a mobile network (not shown), for example to an UMTS LTEnetwork. The radio base station 108 may thus also be referred to as aNode-B.

In order to establish a voice or data call, for example the mobiledevice 102 has to get access to the base station 108. A random accessprocedure has to be performed for synchronizing the device 102 with thetransmission scheme provided by the base station 108 for the radiointerface 110. The random access procedure includes the transmission ofan access burst schematically indicated in FIG. 1 by arrow 112. Theaccess burst is received at the radio base station 108 as a part of aradio signal via the antenna 114. The radio signal may also comprisetransmission bursts from the other mobile devices 104, 106 and furthermobile devices (not shown) in and around the network cell served by thebase station 108. For example, one or both of the devices 104, 106 mayas well transmit an access burst in the RACH of the radio interface 110.

An intermediate stage component 116 distributes the received radiosignal to various further components of the base station 108 and maypossibly provide further functions such as (pre-) filtering of the radiosignal. In particular, the signal may be distributed to a detector 118which is specifically adapted to detect one or more access bursts in therandom access channel provided over the radio interface 110. Thedetector 118 may provide its detection results to further components(not shown) of the base station 108 to trigger a response to the one ormore of the mobile devices 102-106 from which the one or more detectedaccess bursts originate.

The detector 118 is adapted to simultaneously detect multiple accessbursts in a RACH, as will be described in more detail below. Inparticular, the detector 118 may utilize a correlation preamble of aparticular preamble set, the preambles of which may be included by themobile devices 102-106 in the transmitted access bursts.

More than one preamble set may be provided for the radio interface 110.In the example illustrated in FIG. 1, the mobile devices 102-106 mayutilize a preamble of a second preamble set. The specific preamble usedfor a random access procedure may be selected randomly by a mobiledevice. Alternatively, a particular preamble to be used for accessrequests may be implemented, e.g., at the time of manufacture of thedevice. The base station 108 may include a second detector 120 adaptedto detect access bursts utilizing preambles of the second preamble set.The complete RACH radio signal received by the antenna 114 may bedistributed to both the first and the second detectors 118 and 120.

FIG. 2 schematically illustrates functional building blocks of anembodiment of a detector 200, which is adapted for detecting one or moreaccess bursts in a random access channel. The detector 200 may be animplementation of the detector 118 or 120 of FIG. 1.

The detector 200 comprises a reception component 202, which is adaptedto receive, in the random access channel, the one or more access burstseach including its transmission preamble. Each access burst may includea transmission preamble, which is a member of a preamble set includingsequences of preamble symbols that can be obtained by cyclicallyshifting a base preamble sequence. The receiver 202 may receive acomplete signal as it is received at an antenna of a base stationincluding a range of frequencies and channels, or may receive a filteredsignal which includes a RACH only. Additionally or alternatively, thereceiver 202 may comprise its own filter, for example a bandpass-filterfor filtering the frequency range of the random access channel from thereceived signal.

The detector 200 may further comprise a correlation component 204, whichis adapted to determine correlation information indicative of acorrelation of a single correlation preamble with each of the one ormore received transmission preambles, wherein the correlation preambleis any member of the preamble set. Triggered by the random access signalforwarded from the component 202, the correlation component 204 mayaccess a storage 206, wherein the correlation preamble is stored. Thestorage 206 may be external or internal to the detector 200. An externalstorage 206 may belong to a base station within which the detector 200is implemented. The storage 206 may store multiple correlationpreambles, one for each preamble set which may be utilized. Which one ofthe correlation preambles to utilize may be indicated to the correlationcomponent 204, e.g., by a control component of the base station (notshown).

The correlation component 204 may be adapted to treat the correlationpreamble and the one or more transmission preambles included in thereceived random access channel signal as periodic signals. For instance,the correlation component may cyclically correlate the signals. Thecyclic correlation may be performed in a frequency-domain. The resultingcorrelation information includes an indication of a correlation of thecorrelation preamble with one or more transmission preambles (if any)included in the received random access channel signal.

Fast Fourier Transform (FFT) techniques may be used for the correlationprocedure. For example, the correlation component 204 may accessprecompiled FFT- and inverse FFT-procedures, which are stored in astorage component 208 preferably in a precompiled format, fortransforming the received signal and inverse transforming thecorrelation result. The conjugate complex correlation preamble may bestored in the storage component 206 already in a Fourier transformedrepresentation. The correlation of the Fourier transformed receivedsignal with the correlation preamble then requires only somemultiplication and addition operations.

The detector 200 further comprises a detection component 210, which isadapted to detect, based on the correlation information, the one or moreaccess bursts (if any) included in the received radio signal. To thisend the detection component 210 analyses the correlation informationforwarded from the correlation component 204. The detection component210 may, e.g., determine correlation peaks in the correlation function,and may analyse these peaks to determine occurrence of one or moreaccess bursts in the received signal. The detection component 210 mayprovide its detection result(s) to other components of the detectorand/or the base station. For example, the detector 200 may performfurther operations on the received signal such as analysing the detectedaccess burst(s) for an identification of the originating mobiledevice(s).

FIG. 3 illustrates a further embodiment of a detector 300, which may bean implementation of the detector 118 or 120 of FIG. 1. A received radiosignal is provided to an FFT component 304, which transforms thereceived signal into the frequency domain by applying a Fouriertransformation. In a component 306, a frequency range of a RACH isextracted from the frequency-transformed received signal. The resulting,transformed RACH signal is then multiplied with the frequency responseH_(preamble) (f) in the frequency domain at component 308. H_(preamble)represents a conjugate complex Fourier transformed correlation preamble.This multiplication in the frequency domain corresponds to a cycliccorrelation in the time domain.

The correlation result (i.e. the correlation information) is thentransformed back into the time domain by an IFFT (inverse FFT) component310. The output of the IFFT-component 310 is a periodic correlationfunction representing the RACH correlation signal cyclically convolvedwith the channel response. Finally, a detection component 312 detectspreamble sequences in the received RACH signal by analysing thecorrelation information.

Thus, instead of applying a set of matched filters in the time domain toa received RACH signal and calculating an a-periodic correlation, thedetector embodiments described with reference to FIGS. 2 and 3 calculatea periodic correlation.

FIG. 4 schematically illustrates an embodiment of a method 400 fordetecting one or more access bursts in the random access channel. Eachaccess burst includes a transmission preamble, wherein the transmissionpreamble is a member of a preamble set including sequences of preamblesymbols that can be obtained by cyclically shifting a base preamblesequence.

In step 402, the method starts by receiving in the random access channelone or more access bursts, wherein each burst includes its transmissionpreamble. In step 404, correlation information is determined, which isindicative of a correlation of a single correlation preamble with eachof the one or more received transmission preambles. The correlationpreamble is any member of the preamble set. In step 406, the one or moreaccess bursts are detected based on the correlation informationdetermined in step 404. The method ends in step 408 with providing thedetection results to other components of a detector or a radio basestation.

FIG. 5 is a schematic illustration of an embodiment of a time-frequencymapping 500 of a physical random access channel (PRACH). Time andfrequency respectively extend horizontally and vertically. Time slots502 each have a duration T_(RACH)=0.5 ms (milliseconds). A radiotransmission frame of duration 10 ms thus comprises time slots(sub-frames) 0-19. In another embodiment, time slots may have a durationT_(RACH)=1.0 ms each. A radio transmission frame of duration 10 ms wouldthen comprise time slots 0-9. The timeslot duration T_(RACH) may beextended for large cells. The PRACH 504 is assigned the time slot 0(reference numeral 506) in each transmission frame. Any other timeslotmay be assigned as well.

The PRACH may occupy the entire bandwidth BW available over the radiointerface, to which the scheme illustrated in FIG. 5 is applied.However, in the embodiment 500, the bandwidth BW_(RACH) assigned to theRACH is only a fraction of the available bandwidth. Frequency hopping isapplied to the PRACH, such that a different frequency range is assignedto the PRACH in subsequent transmission frames. The RACH is orthogonalto data transmissions in other channels, e.g. traffic channels.

The transmission scheme illustrated in FIG. 5 may be announced to mobiledevices by the base station, e.g., by transmitting related informationin a broadcast channel into the cell served by the base station.

FIG. 6 illustrates the structure of an access burst 600 which may betransmitted by a mobile device in the RACH illustrated in FIG. 5. Theaccess burst 600 comprises a preamble sequence 602. Further sections ofthe access burst (not shown) may represent data related to anidentification of the originating mobile device, the type of connectionrequested by the mobile device, etc.

As the mobile device transmitting the access burst 600 is not timesynchronized in the uplink, the burst arrives at the base station withan unknown propagation delay. Therefore, a guard period (GP) of lengthT_(GP) is required to avoid overlapping of the access burst with otherbursts in subsequent time slots. As an example, the guard period GP mayhave a duration T_(GP)=100 μs (microseconds), such that the access burstmay have a length of 400 μs. In an alternative embodiment, whereinT_(RACH)=1.0 ms, the access burst may have a length of 900 μs. A guardperiod GP with T_(GP)=100 μs leads to a maximum allowed cell radius oforder 15 kilometers. The preamble (having a length of, e.g., 800 μs) canbe received at the base station with a maximum delay of 100 μs.

A cyclic prefix may also be included in the access burst in conjunctionwith the preamble 602. For example, the preamble 602 may be a sequenceof symbols S0- . . . -S7, then a cyclic prefix may comprise symbols S5,S6, S7 which are copied from the end of the preamble and may be appendedto the front of the preamble. Alternatively, the preamble 602 may nothave a cyclic prefix appended, but the base station may copy a portionof the received signal which potentially contains a preamble sequenceand may add the copied portion to the front of the preamble. In thisway, the preamble may be periodically extended.

FIG. 7 schematically illustrates a preamble set 700 based on sixdifferent segments S10-S15. Each of the segments consists of a number ofsymbols; for example, a segment may comprise 1 symbol, 10 symbols, orany other number of symbols. The number of symbols per segment may belarger than 10 symbols. In one embodiment, for example, each of thesegments may comprise 300 symbols. Assuming that each of the segmentscontains one symbol, each of the reference numerals S10-S15 indicatesone symbol.

The sequence length of the preambles of set 700 is 6 times the length ofa single segment. Therefore, in total 6 different preambles can beformed, where all of the sequences are mutually cyclic shifted. Such asmall number of preambles may be sufficient in order to enable multipledevices to simultaneously perform a random access procedure in a cell. Amobile device may randomly select one of the sequences from the preambleset or may use a fixed transmission preamble, which may have beenpre-installed, for example. In some embodiments, instead of only one setof preambles multiple preamble sets may be provided for use by themobile device. The device may then choose one of the preamble sets and aparticular one of the preambles from the chosen preamble set as thetransmission preamble.

The set of preambles 700 illustrated in FIG. 7 might be constructed by(arbitrarily) defining a basic sequence, which is associated with theshift index I=0 in FIG. 7. The basic sequence may be chosen such that itpossesses good or even essentially ideal periodic auto-correlationproperties. In general, the preambles of a preamble set need to haveperiodic (auto-)correlation properties which allow a detection ofcorrelation peaks with at least a desired confidence.

The further preambles of the preamble set may then be constructed bycyclically shifting the symbols forming the basic preamble sequence. Thepreamble sequence with shift index I=1 is constructed by cyclicallyshifting the symbols of the basic preamble sequence clockwise, whereinthe shift amounts to one segment (in this embodiment). The furtherpreamble sequences are constructed similarly. Therefore, assuming that asegment comprises one symbol, the cyclic shift amounts to one symbol. Inan embodiment with the segments comprising ten symbols each, the shiftamounts to ten symbols.

In general, a minimum cyclic shift might be defined which is equal to orlarger than a (cyclic) shift amount corresponding to the maximum allowedpropagation delay in a cell plus the delay spread of the channel. Theminimum shift might amount to one, two or more symbols to be shiftedfrom one sequence to the next in the preamble set. The preamblesequences may be constructed by cyclically shifting integer multiples ofthis minimum shift, starting from the basic preamble sequence. Thisensures that ambiguities in the preamble detection due to propagationround trip time and channel spread can be avoided, as will become clearbelow.

FIG. 8 illustrates the periodic cross-correlation function (CCF) interms of “correlation power” vs. time between preamble sequences of apreamble set and the basic preamble sequence in an ideal case, in whichthe preamble sequences have ideal periodic auto-correlation properties(i.e. the auto-correlation is a Dirac function in the origin). Further,any channel impact is neglected.

The periodic CCF between cyclic shifted preamble sequences with, forexample, shift index I=0, 1, 5 and the basic preamble sequence may leadto correlation functions as indicated in FIG. 8. In particular, thecorrelation function for the shift index I=0 is the auto-correlationfunction of the basic preamble sequence. A cross correlation ofdifferent preamble sequences of the same preamble set with the basicpreamble sequence in the correlation power-time diagram appears shiftedin the time dimension, wherein the time shift is related to the shiftindex I. This can be understood by considering the preamble sequences ofdifferent index I periodically extended in the time dimension. Asequence with a particular index can then be matched to the basicpreamble sequence by shifting it by integer multiples of the shiftamount. The shifting corresponds to a time delay as indicated by thecorrelation functions 802 and 804 in FIG. 8.

FIG. 9 schematically illustrates an embodiment of a preamble sequence900 comprising segments S0-S5, each segment containing one or moresymbols. It is assumed that the transmission preamble 900 is correlatedin a detector such as that of FIGS. 2, 3 with a correlation preamblewhich is the basic preamble of the set including preamble 900. Thecyclic shift between the transmission preamble and the correlationpreamble is 2 times a minimum shift, i.e. shift index I=2. Further,influences of delay spread in the channel are neglected and it isassumed that the preamble 900 (or any preamble of the underlyingpreamble set) comprises ideal periodic autocorrelation properties asillustrated in FIG. 8.

Two cases a) and b) are illustrated in FIG. 9. In case a) it is assumedthat the mobile device transmitting the preamble 900 is located near tothe base station wherein the detector is located. Then the round tripdelay is small and the correlation peak 902 is located within the I=2zone near to its inner border. In contrast, in case b), it is assumedthat the user equipment is located near to the cell border. Consequentlythe round trip delay is large and the correlation peak 904 is locatednear the opposite zone border.

The calculated correlation will have its maximum correlation peak at atime instance corresponding to the amount of cyclic shift plus thepropagation round trip time between mobile device and base station. Thisis the reason why it is preferable that preamble sequences of a preambleset may be shifted against each other by at least a minimum shift, whichamounts to at least the maximum propagation round trip time in the cellplus channel spread. Otherwise, ambiguities may occur in casetransmission preambles with different shift indices and different roundtrip time delays interfere.

The position of the correlation peaks inside the zones may thus be usedto determine the round trip time. This can be done by measuring the timedelay between the inner border of the zone, in which the correlationpeak is located, and the position of the correlation peak. Furthermore,as the correlation peaks of preambles with different shift indices arelocated in different zones of the periodic CCF, multiple transmissionpreambles of a preamble set lead to multiple correlation peaks of thecorrelation function. Thus a simultaneous detection of multiplepreambles with different cyclic shifts can be performed. In other words,a single detector utilizing a single correlation preamble is sufficientfor the detection of multiple access bursts.

The techniques proposed herein thus allow detecting multipletransmission preambles simultaneously with a single detector. As thedetection is based on a single correlation preamble only, the detectormay have a comparably simple structure. Furthermore, calculatingcorrelation information for detecting the access burst(s) may be done byusing FFT techniques which allow a fast and resource-efficientcalculation.

The proposed techniques also allows, following the detection of thepreamble of an access burst, to determine the propagation round triptime in a simple manner. The proposed techniques further allow toarrange two or more detectors in a radio base station, wherein eachdetector utilizes different preamble sets. In this way, the number ofavailable transmission preambles can easily be increased.

While the current invention has been described in relation to itspreferred embodiments, it is to be understood that this disclosure isfor illustrative purposes only. Accordingly, it is intended that theinvention be limited only by the scope of the claims appended hereto.

The invention claimed is:
 1. A method for detecting multiple accessbursts in a random access channel, each access burst including atransmission preamble that is a member of a preamble set, and whereinthe preamble set includes sequences of preamble symbols obtained bycyclically shifting a basic preamble sequence, the method comprising thesteps of: receiving multiple access bursts in the random access channelincluding as transmission preambles different members of the preambleset; determining correlation information comprising a correlationfunction indicating a correlation between a single correlation preambleand each of the received multiple different transmission preambles in atime dimension, wherein the correlation preamble is a member of thepreamble set, and wherein determining the correlation informationfurther comprises subdividing the correlation function into zones, eachzone being associated with the correlation of one member of the preambleset with the correlation preamble; and substantially simultaneouslydetecting the multiple access bursts by analyzing the correlationinformation determined using the single correlation preamble.
 2. Themethod of claim 1 wherein determining the correlation informationcomprises treating the correlation preamble and the multipletransmission preambles as periodic signals.
 3. The method of claim 2wherein determining the correlation information further comprisescyclically correlating the correlation preamble and the multipletransmission preambles.
 4. The method of claim 3 wherein cyclicallycorrelating the correlation preamble and the multiple transmissionpreambles is performed in a frequency domain.
 5. The method of claim 3wherein cyclically correlating the correlation preamble and the multipletransmission preambles is performed using Fourier Transform techniques.6. The method of claim 5 further comprising: performing a FourierTransform of a received signal; and extracting a frequency bandcorresponding to the random access channel.
 7. The method of claim 1further comprising determining, based on the correlation information, apropagation round trip time for at least one of the multiple receivedtransmission preambles.
 8. The method of claim 1 wherein the sequencesof the preamble set are cyclically shifted versions of each other, andare shifted by at least a minimum shift corresponding to a predeterminedmaximum time delay for the transmission preambles depending on channelpropagation properties.
 9. The method of claim 1 wherein each preamblesymbol utilized for the preamble sequences is utilized only once perpreamble sequence.
 10. The method of claim 1 wherein determining thecorrelation information further comprises: determining a correlationpeak in the correlation function corresponding to one of the multipletransmission preambles; determining the zone within which thecorrelation peak is located; and detecting one of the multiple receivedaccess bursts based on the determined zone.
 11. The method of claim 10further comprising: determining a peak shift of the correlation peakrelative to a border of the zone within which the correlation peak islocated; and calculating, based on the peak shift, the propagation roundtrip time.
 12. A non-transitory computer readable medium comprisingprogram code portions stored thereon, the code configured to control acomputing device to detect multiple access bursts in a random accesschannel, each access burst including a transmission preamble that is amember of a preamble set, wherein the preamble set includes sequences ofpreamble symbols obtained by cyclically shifting a basic preamblesequence, the code configured to further control the computing deviceto: receive multiple access bursts in the random access channelincluding as transmission preambles different members of the preambleset; determine correlation information comprising a correlation functionindicating a correlation between a single correlation preamble and eachof the received multiple different transmission preambles in a timedimension, wherein the correlation preamble is a member of the preambleset, and wherein the code is further configured to determine thecorrelation information by subdividing the correlation function intozones, each zone being associated with the correlation of one member ofthe preamble set with the correlation preamble; and substantiallysimultaneously detect the multiple access bursts by analyzing thecorrelation information determined using the single correlationpreamble.
 13. A detector for detecting multiple access bursts in arandom access channel, wherein each access burst includes a transmissionpreamble, the transmission preamble being a member of a preamble setincluding sequences of preamble symbols obtained by cyclically shiftinga basic preamble sequence, the detector comprising: a receptioncomponent configured to receive multiple access bursts in the randomaccess channel including as transmission preambles different members ofthe preamble set; a correlation component configured to determinecorrelation information comprising a correlation function indicating acorrelation between a single correlation preamble and each of thereceived multiple different transmission preambles in a time dimension,wherein the correlation preamble is a member of the preamble set, andwherein the correlation component is further configured to determine thecorrelation information by subdividing the correlation function intozones, each zone being associated with the correlation of one member ofthe preamble set with the correlation preamble; and a detectioncomponent configured to substantially simultaneously detect multipleaccess bursts by analyzing the correlation information determined usingthe single correlation preamble.
 14. The detector of claim 13 whereinthe correlation component is configured to treat the correlationpreamble and the multiple transmission preambles as periodic signals.15. A radio base station for performing a random access procedure, theradio base station comprising: a detector configured to substantiallysimultaneously detect multiple access bursts in a random access channel,each access burst including a transmission preamble that is a member ofa preamble set including sequences of preamble symbols obtained bycyclically shifting a basic preamble sequence, the detector comprising:a reception component configured to receive, in the random accesschannel, multiple access bursts including as transmission preamblesdifferent members of the preamble set; a correlation componentconfigured to determine correlation information comprising a correlationfunction indicating a correlation between a single correlation preambleand each of the received multiple different transmission preambles in atime dimension, wherein the correlation preamble is a member of thepreamble set, and wherein the correlation component is furtherconfigured to determine the correlation information by subdividing thecorrelation function into zones, each zone being associated with thecorrelation of one member of the preamble set with the correlationpreamble; and a detection component configured to substantiallysimultaneously detect multiple access bursts by analyzing thecorrelation information determined using the single correlationpreamble.
 16. The radio base station of claim 15 further comprising twoor more detectors, each detector being configured for a differentpreamble set.